Field of the Invention
The present invention generally relates to devices whose electrode is electrically connected to a through wire, such as capacitive transducers used as ultrasonic transducing devices and the like, and methods for manufacturing the devices. More specifically, the present invention relates to a capacitive transducer including a through electrode on a substrate, and a method for manufacturing the capacitive transducer.
Description of the Related Art
Capacitive micromachined ultrasonic transducers (CMUTs) have been hitherto investigated as a replacement of piezoelectric devices. CMUTs have capabilities to transmit and receive an acoustic wave such as an ultrasonic wave by using the vibration of a vibration film, and to easily obtain excellent broadband characteristics particularly in liquids. In practice, a plurality of elements, each of which is composed of a plurality of vibration films (or cells) arranged in a two-dimensional array, are arranged on a substrate to form a single device, thereby implementing the desired performance. Independent control of the respective elements requires connection wires to be formed, each corresponding to one of the elements.
In the structure described above, it is desirable to use a through wire or through electrode extending through a substrate to reduce the size of a device and the parasitic capacitance of a connection wire. Methods for manufacturing a device that includes a through wire include a “via first” approach and a “via last” approach. In the via-first approach, a through wire that extends through a substrate is formed prior to the formation of a device. In the via-last approach, in contrast, a through wire that extends through a substrate is formed after the formation of a device. The via-first approach may be desirable in terms of device performance or manufacture.
U.S. Patent Application Publication No. 2007/0264732 discloses a CMUT that includes a through wire. The disclosed CMUT is manufactured using a via-first approach, and polysilicon is used as a material of the through wire. The through wire composed of polysilicon has relatively high process resistance in the manufacturing process after the formation of the through wire, and also has relatively high environmental resistance in the use of the CMUT. Japanese Patent Laid-Open No. 2010-45371 discloses a through-electrode structure having a conductive through wire. The conductive through wire has a conductive protective film on a bottom thereof to prevent the conductive through wire from being subjected to surface oxidation and damage.
In the CMUT disclosed in U.S. Patent Application Publication No. 2007/0264732, the through wire is composed of high-resistivity polysilicon, and it is not easy to reduce the resistance of the through wire. As a result, a reduction in the device characteristics of the CMUT is likely to occur. A through wire composed mainly of a low-resistance metal (such as Cu) is preferable in terms of the device characteristics of the CMUT. In the through-electrode structure disclosed in Japanese Patent Laid-Open No. 2010-45371, a through wire composed of a conductive material is used to reduce the resistance of the through wire. On the other hand, the complexity of the manufacturing process for the through wire is high. In addition, a portion of the through wire is exposed through the protective film. If a device such as a CMUT is fabricated by via-first using the through-wire structure described above, the exposed portion of the through wire may be chemically or mechanically damaged in the fabrication process. As a result, the through wire is prone to developing surface roughness or a reduction in length, which may not facilitate reliable and low-resistance connection with a lead wire. In general, reduced chemical or mechanical damage requires a limitation on the material of the through wire or an increase in the number of steps in the device manufacturing process. Such a requirement is likely to lead to a reduction in device performance or an increase in manufacturing costs. In light of such technical issues, it is desirable to easily manufacture a low-resistance high-chemical-resistance through-wire structure to maintain device performance and reduce manufacturing costs.